The technology at the MPG HLL is optimized for fully depleted double-side processed radiation sensors. At the same time, many of our sensors (like DEPFET or pnCCD) have also the first amplification stage for charge-to-current or charge-to-voltage conversion monolithically integrated in the sensor silicon. For sensor control and biasing, further signal processing, and off-sensor data transmission, an electrical interconnect to auxiliary electronics is required. These auxiliary building blocks are mostly very sophisticated application specific integrated circuits (ASICs) with functionality and performance specifically adapted to the sensor and the requirements of the application. The ASICs are designed in close collaboration with chip designers of partner universities and fabricated in micro-electronics industry utilizing the best available fabrication technology.
The ASICs are either placed on high-density interconnect boards (ceramic or organic substrates) or directly on the silicon of the sensors substrate forming a complex multi-chip module with sensing capabilities, analogue signal processing and digital control blocks on one single piece of silicon. Depending on the number of read-out nodes of the sensor, the speed requirements in the application, and the level of parallelization of the read-out, there can be up to a few thousand connections between the sensor and the ASICs.
These interconnects are realized by micro-wire bond connections or fine-pitch solder connections with solder balls (“bumps”) deposited on the sensor or ASIC side prior interconnect process. The latter process is also called “flip-chip” as the chips have to be flipped and attached face down to the sensor.
Wire bonding is a micro-welding technique utilizing ultrasonic or thermosonic agitation of an ultra-fine aluminium or gold wire to be mechanically and electrically attached to bonding pads on sensor and read-out ASIC. The micro-wires can be as fine 17 µm in diameter, which is about five times finer than a human hair. Flip-Chip interconnects are basically solder joints between the sensor and the ASICs. While for wire bonding bonding pads have to be placed along the rim of the chips, with flip-chip, the bumps can be placed over the entire surface of the ASIC utilizing a so-call re-distribution layer of additional metal. The industrial standard for the minimal 2D interconnect pitch is about 150 µm, at HLL assemblies with 50 µm as interconnect pitch have been already achieved and we are constantly improving our capabilities to reduce the pitch and hence the density of flip-chip interconnections further.